Cation exchange resins are widely used for removing ionic species and particulates from water streams. A typical resin includes a copolymer, such as a styrene-divinylbenzene copolymer, to which sulfonic acid anionic sites have been attached by a sulfonation reaction.
Useful cation exchange resins contain essentially no leachable compounds, organic or inorganic, that might leave the resin and contaminate the water stream. Additionally, the cation exchange resin must possess sufficient hydraulic stability to resist compressive forces exerted by a flowing stream of water and substantially maintain its shape.
In order to provide such useful cation exchange resins, resin designers have adjusted hydration, swelling, and porosity of cation exchange resins through proper choice of polymers and copolymers and through control of the degree of cross-linking of polymer chains. Additionally, resins having pores with dimensions significantly larger than the molecular distance between adjacent copolymer chains have been synthesized by the inclusion of various uncrosslinked polymers in the monomer mixture which are rendered soluble and leachable following sulfonation. The leachable polymers are removed, leaving relatively large-sized pores throughout the cation exchange resin. Other macroreticular resins have been synthesized by polymerizing a resin in a solvent which dissolves monomer reagents but exerts essentially no solvent action on the copolymer produced.
Cation exchange resins have been used to purify many different types of water streams, ranging from household drinking water to industrial waste. With the development of modern high-pressure steam generation systems, cation exchange resins are currently employed to treat boiler feedwater makeup streams and recycle condensate water streams. Condensate polishing units containing cation exchange resins are used to remove impurities, such as iron oxide corrosion products, from condensate streams produced in chemical manufacturing processes and in electrical power generating plants, both fossil-fueled and nuclear-powered.
Condensate polishing units containing cation exchange resins are the norm for modern steam generation systems operating at 2000 psig. or more. The use of condensate polishing units leads to improved turbine efficiency, shorter unit startup time, protection from the effects of condenser leakage, and longer intervals between acid cleanings. The cation exchange resins may be present in relatively deep beds or in coatings on filters.
The use of cation exchange resins to purify condensate streams in boiling water reactor (BWR) nuclear reactors is illustrative. In a BWR system, water is circulated through a nuclear reactor core producing saturated steam which is directed to a steam turbine generator and, thereafter, passed to a condensate polishing unit. The steam itself is not radioactive, although impurities in the steam, such as iron oxide and copper oxide particulate, may be converted to radioactive isotopes. As in fossil-fired power plants, improved turbine efficiency and protection from an accumulation of deposits in the steam system are important considerations. Consequently, cation exchange resins are employed in condensate polishing units to remove metal oxide particles which are often present in BWR condensate in the range of about several parts per billion by weight. Cation exchange resins have been developed specifically for this application. These cation exchange resins remove the insoluble iron and copper oxides by means of adsorption and filtration.
More recently, it has been noted that some of the ion exchange resins used for purifying nuclear BWR condensate streams demonstrate better iron removal efficiency after several months of service. Experts theorize that a fraction of the divinylbenzene linkages, which cross-link styrene polymer chains in the resins, yield to oxidative attack during service. The resins with a reduced degree of cross-linking appear to remove iron from condensate more efficiently. Accordingly, cation exchange resins were developed which contain a relatively low degree of divinylbenzene cross-linking between styrene polymer chains. The cation exchange resins with relatively low degree of cross-linking have been tested in at least one condensate polishing unit serving a nuclear BWR reactor and increased iron removal efficiency has been reported.
However, the testing of new cation exchange resins on a relatively large pilot plant scale is both expensive and time-consuming. Many types of conventional ion exchange resins are available to steam system operators for removing iron oxides from condensates. From the myriad of products available, steam system operators attempt to select ion exchange resins that can be used to safeguard and improve steam system performance.
Unfortunately, few guide lines exist for selecting an ion exchange resin capable of removing iron oxides from condensate to the very low levels required for boiler feed water in high-pressure steam systems. Selecting a resin by trial and error is a task of elephantine proportions, since there are no known methods for performing such studies in the laboratory. Further, there are no known methods for preparing simulated condensates containing iron oxides having the correct chemical and physical properties for bench-scale laboratory testing. At the current state of the art, tests must be conducted on a relatively large pilot plant scale, usually on the site of an industrial operation.
Although conventional ion exchange resins have proven useful for condensate polishing in the past, a need exists for a cation exchange resin having an enhanced capacity for iron oxides, especially in a mixed bed mode of operation. A desirable resin would demonstrate removal efficiencies of 90% or more and produce a product stream that contained only a few parts per billion by weight of iron oxides, in conformance with limits set by the nuclear BWR industry. An ideal cation exchange resin would have a very high specific surface area in order to adsorb as much iron as possible. Also, the rate of diffusion of ferric and ferrous irons into the resin structure would be high so that surface ion exchange sites would remain in an active hydrogen form, allowing uninterrupted adsorption and dissolution of the iron oxides.